Instruction manual

80 McQuay OM 920
Operator’s Guide
Energy Recovery
Energy recovery is provided by drawing outside air across half of an Enthalpy Wheel and
drawing exhaust air across the other half. Latent and sensible heat is transferred from the
hotter, moister exhaust air to the colder dryer outside air in winter. Latent and sensible heat is
transferred from the hotter more moist outside air to the cooler dryer air exhaust in summer.
Control consists of starting and stopping an exhaust fan, modulating the speed of the exhaust
fan, starting and stopping the Enthalpy Wheel, and optionally controlling the speed of the
Enthalpy Wheel. The outdoor dampers are controlled in the normal manner. The current
statuses as well as editable parameters associated with Energy Recovery are located in the
Standard Menus/Energy Recovery menus.
Enthalpy Wheel
The Enthalpy Wheel is turned on when all of the following are true.
• The Exhaust Fan is on
• The OA Dampers are at the minimum position
• The unit is not in the Economizer State
• The wheel has not been turned off due to Frosting/Condensation
Note – When turned on the speed of the enthalpy wheel is set to the maximum speed for units
with a VFD controlling the speed of the Enthalpy Wheel.
The Enthalpy Wheel is turned off when any of the following is true
• The Exhaust Fan is off OR
• The OA Damper Position is driven above the Minimum OA Damper Position by more than
3%
Additional control is provided for units with a VFD controlling the speed of the Enthalpy
Wheel to prevent Frosting/Condensation on the Enthalpy Wheel. This control is based on an
Intersection Point described in paragraphs that follow.
The Enthalpy Wheel VFD speed is set to zero so it will operate at its internally edited
minimum speed when both of the following are true:
• The wheel has been operating at maximum speed for longer than the edited Wheel Stage
Time
• The exhaust air temperature leaving the Enthalpy wheel is below the Intersection Point plus
an edited Minimum Temperature Difference
To prevent Frosting/Condensation on the Enthalpy Wheel, the Enthalpy Wheel is turned off
when both of the following are true:
• The exhaust air temperature leaving the Enthalpy wheel is below the Intersection Point plus
an edited Minimum Temperature Difference
• The wheel has been operating at its internally edited minimum speed for longer than an
edited Wheel Stage Time
The Enthalpy Wheel is turned on at minimum speed when:
• The Wheel has been off due to Frosting/Condensation for longer than an edited Minimum
off Time
• The exhaust air temperature leaving the Enthalpy wheel is above the Intersection Point plus
an edited Maximum Temperature Difference
The Enthalpy Wheel speed will be increased to its maximum speed when:
• The exhaust air temperature leaving the Enthalpy wheel is above the Intersection Point plus
an edited Maximum Temperature Difference
• The Wheel has been operating at Minimum Speed for longer than the edited Wheel Stage
Time